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Current issues in pion absorption by nuclei around the Δ resonance are discussed. The reaction strength nas little dependence on the nuclear density. Final state interaction losses are understood, but the apparent weakness of the initial state interaction probability is a major issue. Evidence for a 3N absorption dynamics is discussed.

A plastic scintillation counter used to determine the energy of positive pions and protons is described. The measured energy resolution and efficiency for 50 MeV pions are 1 MeV and 87% respectively. Fast response permits high-count-rate operation and pion identification. This stopping counter is useful for positive pions of energies up to about 100 MeV.

Angular distributions were measured for π + and π − scattering from 16 O and 18 O at 163 MeV. The π + elastic cross sections are similar but significant differences are observed between the π − elastic cross sections confirming the expected greater sensitivity of the π − to the 2 extra neutrons in 18 O. Appreciable differences between π + and π − are also observed for the inelastic scattering to the 2 + state at 1.98 MeV and the 3 − state at 5.09 MeV.

Inelastic pion scattering from $^{16}\mathrm{O}$ has been studied by measuring the spectrum of pions down to 40 MeV at five angles between 30\ifmmode^\circ\else\textdegree\fi{} and 134\ifmmode^\circ\else\textdegree\fi{} for three incident pion energies: 114, 163, and 240 MeV. The spectra are dominated by a broad peak due to quasielastic scattering from a single nucleon in the nucleus. The systematics of the spectra are discussed with emphasis on the pion-nucleus interaction dynamics including the effect of absorption. The partial ${\ensuremath{\pi}}^{+}$-$^{16}\mathrm{O}$ cross sections for all nonradiative channels are estimated.NUCLEAR REACTIONS $^{16}\mathrm{O}(\ensuremath{\pi}, {\ensuremath{\pi}}^{\ensuremath{'}})$ $E=114, 163, 240$ MeV. $\ensuremath{\theta}=30\ensuremath{-}134\ifmmode^\circ\else\textdegree\fi{}$, full inelastic spectrum measured; $\ensuremath{\sigma}(\ensuremath{\theta})$. Discussion of nuclear medium effects on quasifree processes, $\ensuremath{\Delta}$ propagation.

The current status of pion-nucleus interactions is reviewed with an emphasis on recent experimental results.

The elastic and inelastic (3 − , 2.6 MeV) scattering of pions by 208 Pb has been measured at 80 MeV ( π + only) and 116 MeV (π + and π − ). An optical potential model including quadratic density terms reproduces well the transition from diffractive scattering at the higher energies to the non-diffractive behavior observed at 50 MeV. Good agreement with the inelastic data is obtained with a collective model using the same deformation length βR = 0.85 fm ( β = 0.13) for π + and π − .

The scattering of positive pions by 16O has been studied using a magnetically analysed beam of 385 MeV/c pions and a magnetic spectrometer for the scattered pions. Full inelastic spectra were recorded at four angles and the elastic scattering at twenty angles. The elastic scattering is compared with calculations made in the Glauber approximation. The Glauber calculations fit the experimental data extremely well.

Cross sections for reactions of the type pA → (A+1)π+, where A and (A+1) are bound nuclei, have been expressed in terms of the cross section for the reaction pp → dπ+ and a form factor, through an approximation originally due to Ruderman. The formula successfully accounts for the energy dependence of bound state pion production and, roughly, for the magnitude of the cross sections.

The SIN pion spectrometer has been built to carry out a broad range of experiments on pion‐nucleus interactions in the momentum range of 150‐650 MeV/c. It is designed to subtend a large solid angle (16 msr), with a large momentum acceptance (±18%‐p/p), and with an overall momentum resolution, including the contribution from the pion channel, of 7×10−4 dp/p FWHM. At present an overall resolution of 1.3×10−3 dp/p has been achieved, over a momentum acceptance of ±7.5% ‐p/p.

Inelastic scattering of 163- and 241-MeV positive pions by /sup nat/Ca has been studied with special attention paid to the giant-resonance region. Pronounced giant resonance structures are observed at both incident energies. The analysis indicates a strong excitation of the giant quadrupole resonance at 18.2 MeV (GAMMA approx. 3--3.5 MeV).

The optical properties of the SIN high resolution pion channel and spectrometer are briefly reviewed. The operation of the whole system, including detectors and data acquisition is then described. Finally the tune-up method and the actual performances are given.